ALKENES
STRUCTURE . . . REACTIVITY
Alkene
Hydrocarbon With Carbon-Carbon Double Bond
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Also called an olefin but alkene is better
Unsaturated HC
Has less no. of carbons than the corresponding alkanes
Includes many naturally occurring materials
• Flavors, fragrances, vitamins
STRUCTURE
Ethylene
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Ethylene, or ethene, is one of the most valuable
products obtained from petroleum refining.
Ethylene is the starting material for polyethylene
which is found in a variety of commercial
products including milk bottles and plastic bags.
Ethylene is also a natural product which is
released when fruits ripen.
It is ethylene that is released from ripening and
rotting fruits that causes one bad apple to spoil
the
barrel.
STRUCTURE
Propene
• Propene, commonly known as propylene, is
the second member of the alkene family.
• Propylene
is usually produced during
petroleum refining or as a coproduct of
ethylene production.
• Polypropylene is an important commercial
product made from propene and is found in
carpets, ropes, yarns, and fishing gear.
• Propene is named by changing the -ane
ending of the parent alkane with -ene.
STRUCTURE
BUTENE
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Butene (C4H8) can exist as three constitutional isomers.
Two isomers, 1-butene and 2-butene, differ only in the position of
the double bond.
The third butene isomer has a different carbon skeleton and is
named as a propene derivative following the IUPAC rules.
Two forms of 2-butene are
possible depending on the
relative position of the
methyl groups.
These
two
forms
are
geometric isomers.
The isomer in which the two
methyl
groups
are
on
opposite sides of the double
bond is called trans-2-butene.
The other isomer with the
methyl groups on the same
side of the double bond is cis2-butene.
PHYSICAL PROPERTIES
1.
2.
Same as alkanes; non-polar, insoluble in H2O,
low density, b.p. rises 20o-30o for each C added.
Dipole moments are larger than for alkenes due
to the loosely held ᴨ electrons.
INDUSTRIAL PREPARATION & USES
COMPOUNDS DERIVED INDUSTRIALLY FROM ETHYLENE (ETHENE) AND FROM
PROPYLENE (PROPENE)
Industrial Preparation and Use of Alkenes
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Ethylene and propylene are the most
important organic chemicals produced
Made by thermal cracking of light alkanes
(petroleum)
Calculating Degree of Unsaturation
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Relates molecular formula to possible structures
Degree of unsaturation: number of multiple bonds or
rings
Formula for a saturated acyclic compound is CnH2n+2
Each ring or multiple bond replaces 2 H's
Example: C6H10
Saturated is C6H14
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Therefore 4 H's are not present
This has two degrees of unsaturation
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Two double bonds?
or triple bond?
or two rings
or ring and double bond
Degree of Unsaturation With Other
Elements
Organohalogens (X: F, Cl, Br, I)
• Halogen replaces hydrogen
C4H6Br2 and C4H8 have one degree of unsaturation
Organoxygen compounds (C,H,O) - if connected by single bonds
• These don't affect the total count of H's
Organonitrogen compounds
Nitrogen has three bonds
• So if it connects where H was, it adds a
connection point
• Subtract one H for equivalent degree of
unsaturation in hydrocarbon
Summary - Degree of Unsaturation
Method 1
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Count pairs of H's below CnH2n+2
Add number of halogens to number of H's (X equivalent to H)
Ignore oxygens (oxygen links H)
Subtract N's - they have three connections
Method 2
H sat  2C  2  X  N (ignore O, S)
H sat - H act
Degree of Unsaturat ion 
2
Naming of Alkenes
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Name the parent hydrocarbon—change ending to “–ene”
Number carbons in chain so that double bond carbons have
lowest possible numbers
Rings have “cyclo” prefix—double bond always C#1, C#2
Multiple “C=C“ are named as “diene” “triene” “tetraene” etc…
Alkenes higher priority than alkanes: even shorter chain
Many Alkenes Are Known by Common Names
Cis-Trans Isomerism in Alkenes
Carbon atoms in a double bond are sp2-hybridized
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Three equivalent orbitals at 120º separation in plane
Fourth orbital is atomic p orbital
Combination of electrons in two sp2 orbitals of two
atoms forms  bond between them
Additive interaction of p orbitals creates a  bonding
orbital
Occupied  orbital prevents rotation about -bond
Rotation prevented by  bond - high barrier, about 268
kJ/mole in ethylene
Rotation of  Bond Is Prohibitive
This prevents rotation about a carbon-carbon
double bond (unlike a carbon-carbon single bond).
Creates possible alternative structures
The presence of a carbon-carbon double bond can create two
possible structures
• cis isomer - two similar groups on same side of the
double bond
• trans isomer - similar groups on opposite sides
Each carbon must have two different groups for these isomers
to occur
Cis, Trans Isomers Require That
End Groups Must Differ in Pairs
 180°rotation superposes
 Bottom pair cannot be superposed without breaking C=C
Sequence Rules: The E,Z Designation
Neither compound is clearly “cis” or “trans”
• Substituents on C1 are different than those
on C2
• We need to define “similarity” in a precise
way to distinguish the two stereoisomers
Cis, trans nomenclature
disubstituted double bonds
only
works
E/Z Nomenclature for 3 or 4 substituents
for
E,Z Stereochemical Nomenclature
Priority rules of Cahn,
Ingold, and Prelog
Compare where higher
priority groups are with
respect to bond and
designate as prefix
E -entgegen, opposite
sides
Z - zusammen,
together on the same
side
Ranking Priorities:
Cahn-Ingold-Prelog Rules
RULE 1
Must rank atoms that are connected at comparison point
Higher atomic number gets higher priority
• Br > Cl > S > P > O > N > C > H
Extended Comparison
RULE 2
• If atomic numbers are the same, compare at next
connection point at same distance
• Compare until something has higher atomic number
• Do not combine – always compare
Dealing With Multiple Bonds:
RULE 3
• Substituent is drawn with connections shown and no double
or triple bonds
• Added atoms are valued with 0 ligands themselves
Stability of Alkenes
Cis alkenes are less stable than trans alkenes
Compare heat given off on hydrogenation: Ho
Less stable isomer is higher in energy
• And gives off more heat
• tetrasubstituted > trisubstituted > disubstituted > monosusbtituted
• hyperconjugation stabilizes
Hyperconjugation
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Electrons in neighboring filled  orbital stabilize vacant
antibonding  orbital – net positive interaction
Alkyl groups are better than H
Alternative Explanation:
sp3—sp2 bond > sp3—sp3 bond
Electrophilic Addition of Alkenes
General reaction
mechanism: electrophilic
addition
Attack of electrophile (such
as HBr) on  bond of alkene
Produces carbocation and
bromide ion
Carbocation is an
electrophile, reacting with
nucleophilic bromide ion
Electrophilic Addition for preparations
The reaction is successful with HCl and with HI as well as HBr
HI is generated from KI and phosphoric acid
Orientation of Electrophilic
Addition: Markovnikov’s Rule
In an unsymmetrical alkene, HX reagents can add in two
different ways, but one way may be preferred over the other
If one orientation predominates, the reaction is regiospecific
Markovnikov observed in the 19th century that in the addition
of HX to alkene, the H attaches to the carbon with the most H’s
and X attaches to the other end (to the one with the most alkyl
substituents) This is Markovnikov’s rule.
Example of Markovnikov’s Rule
Addition of HCl to 2-methylpropene
Regiospecific – one product forms where two are possible
If both ends have similar substitution, then not regiospecific
Markovnikov’s Rule (restated)
 More
highly substituted carbocation forms as intermediate
rather than less highly substituted one
 Tertiary cations and associated transition states are more
stable than primary cations
Carbocation Structure and Stability
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Carbocations are planar and the tricoordinate carbon is
surrounded by only 6 electrons in sp2 orbitals
The fourth orbital on carbon is a vacant p-orbital
The stability of the carbocation (measured by energy
needed to form it from R-X) is increased by the presence of
alkyl substituents (Hyperconjugation stabilizes C+)
ANALYSIS OF ALKENES
1.
2.
3.
REACTION WITH BROMINE IN CCl4
BAYER’S TEST FOR UNSATURATION
SOLUBILITY TEST